3rd Cyrstal Sword Fight

I can not defeat the 2 "white knights" after I retrieve the 3rd sword. I have leved uped and plenty of health, but still can not defeat them. What is the best method for attack? Can you change difficultly in mid-game?

The way that worked for me was, to be totally aggressive (I used Blade geysers and flip-saw alot) with them to break their armor. But the second you see their weapon glow, dash out of the way 2x, It's unblockable. You'll soon pick up a rhythm and it'll be over before you know it.